Bonded materials are used in a variety of structural applications. For example, adhesively bonded, laminated composite structures are increasingly being used in aircraft construction to reduce weight, reduce or eliminate the number of separate components, and improve fuel efficiency. The presence of material defects in a composite aircraft structure can lead to disastrous failure of the structure under flight loads. These defects may exist in the composite laminate itself, as well as in the adhesive bonds in the structure. The defects may arise as a result of damage during service, or in the original manufacturing process.
The growing ubiquity of composite structures has led to an increased need for techniques to evaluate the strength of the composite structures, including the adhesive bonds themselves, without damaging or destroying the composite structures. Conventional non-destructive evaluation (“NDE”) techniques are useful when a gap, crack, or void is present in a bonded material. However, conventional NDE techniques do not adequately identify deficiencies, such as weak bonds or “kissing bonds,” where materials bonded together are in contact but without adequate structural strength. These deficiencies can result from bond surface contamination, improperly mixed or outdated adhesives, and improper adhesive application.
Laser bond inspection (“LBI”) is an NDE technique for testing the integrity of bonded materials and structures. LBI is a method that involves sending a precisely controlled dynamic stress wave through an adhesive bond of a composite structure. Generally speaking, and with reference to FIG. 1, LBI involves the deposition of laser energy at a first surface of a bonded material, generating a compressive stress wave. The compressive stress wave propagates through the bonded material, through a bond of interest, to a second surface of the bonded material, where the wave is reflected as a tensile wave (not shown). The tensile wave propagates back through the bonded material and, when it reaches the bond of interest, stresses the bond. The application of dynamic stress on the bonded material is low enough to have no effect on the integrity of the bonded material or the bond itself if the bond is sufficiently strong. However, if the bond is below a suitable strength, the tensile wave will cause the bond to fail (or will expose its non-bonded nature, in the case of a kissing bond).
The traditional instrument for capturing surface velocity of high speed targets, such as when bonds are disrupted during LBI, has been the Velocity Interferometer System for Any Reflector (“VISAR”). See, e.g., U.S. Pat. No. 6,848,321 (The Boeing Company), which is incorporated by reference herein in its entirety. However, VISAR instruments are expensive, relatively difficult to use, and require extensive care for proper optical alignment and light collection. More recently, electromagnetic acoustic transducers (“EMAT” gauges) have been used with success in LBI. See, e.g., U.S. Pat. No. 7,770,454 and U.S. Pat. App. Pub. No. 2008/0257048, now U.S. Pat. No. 8,156,811 (LSP Technologies, Inc.), both of which are incorporated herein by reference in their entireties. EMAT gauges are relatively inexpensive and versatile. However, EMAT gauges often must be placed close to the workpiece surface being tested and can be sensitive to the standoff distance. Furthermore, the workpiece generally must have an electrically conductive surface in order to ensure proper operation of the EMAT gauges, or an electrically conductive material must be applied to the surface. The application of an electrically conductive surface to the workpiece can be expensive and cumbersome.
What is needed is an economical, relatively high standoff, non-contact sensor for measuring rapid changes in surface velocity such as those measured by laser bond inspection.